Dropout voltage in a voltage regulator is defined in the art as the smallest voltage difference that may be permitted between the regulator input and output without adversely affecting regulator performance. As is known in the art, a high dropout voltage is undesirable in most regulators because in addition to requiring a high input voltage, it can increase the temperature of the circuit elements, consequently decreasing the life of the regulator. Accordingly, regulators having a high dropout voltage should not be used in low voltage, low power systems.
Conventional voltage regulators that provide a low dropout voltage (i.e., less than about 1.5 volts) typically must be stabilized by coupling a relatively large capacitor to the regulator output. The capacitor stabilizes the circuit by adding a dominant pole and a zero (to cancel a non-dominant pole) to the frequency response of the regulator circuit.
Use of a capacitor across the output of a voltage regulator is undesirable, however, for a number of reasons. Primarily, it is an extra element that both increases the overall cost of the system, and utilizes a relatively large amount of circuit board space. In addition, although used to stabilize the regulator, the capacitor can destabilize the regulator since it relies upon a number of unstable parasitics for proper performance. Moreover, the capacitor limits the regulator to a small bandwidth, consequently slowing the response time of the regulator.
Exemplary voltage regulators having a low dropout voltage include PNP regulators and composite regulators. PNP regulators operate by sampling the regulator output voltage via a feedback loop, and then comparing the output voltage to a reference voltage. The reference voltage typically is a trimmed bandgap voltage of approximately 1.25 volts. Based upon the comparison of these two voltages, a high gain operational amplifier controls the base-emitter voltage of an NPN transistor, which responsively controls the output current from the collector of a PNP driving transistor to drive a load.
Composite regulators operate in a manner similar to PNP regulators since they include both a PNP regulator circuit, as described above, and a high bandwidth emitter follower circuit that acts as an output driver. As is known in the art, however, composite regulators have AC characteristics that are very similar to those of PNP regulators. Accordingly, like PNP regulators, composite regulators have a low dropout voltage and require a capacitor for stabilization purposes.
Unlike PNP regulators and composite regulators, conventional Darlington regulators typically do not require an external stabilization capacitor but do have a high dropout voltage. Accordingly, because of such high dropout voltage, Darlington regulators should not be used with low voltage, low power systems.